Interestingly, I've had fairly strict religious Jews tell me that non-/pre-marital sex is NOT considered a sin under many circumstances according to their religion. They've always told me that the fixation on total abstinence outside of marriage is something Christians invented, and not a part of its Judaic heritage.
It is an interesting cultural note, considering the history.
Then they arent reading the Bible for that point of view.
Out of curiosity, suppose that two brothers, one of whom was first-born, got married and the elder brother were to die. Would Jews today consider it mandatory, optional, or improper that the youngest brother should sleep with his elder brother's widow and attempt to impregnate her?